This invention relates to enzymatic processes for liquefying starch and converting the liquefied starch to glucose (dextrose) which can then be converted to fructose (levulose).
Most food grade glucose is provided as an enzymatic hydrolysate of corn starch, i.e., the corn syrup of commerce. Glucose is generally rated at being 60 to 80% as sweet as surcrose and therefore sells at a correspondingly lower price. It has long been known to isomerize glucose to fructose which is even sweeter than sucrose by employing an enzyme having glucose isomerase activity, preferably one which has been immobilized upon an inert support such as diethylaminoethyl-cellulose, porous glass or chitin. The isomerization of glucose provides an equilibrium mixture typically containing 42-55% fructose and is referred to as high fructose corn syrup (HFCS).
It is known that alpha-amylase, glucoamylase and glucose isomerase can be isolated from a substantial number of bacteria including species of Streptomyces, Bacillus, Acetobacter, Norcardia, Lactobacillus, Ampullariella, and various other genera of bacteria, and the enzymes have been employed in the commercial production of fructose from starch by known enzymatic techniques to provide mixtures of glucose and fructose. In the commercial process most commonly in present use, cornstarch is liquefied, enzymatically or chemically, and then treated with glucoamylase to produce glucose which is thereafter isomerized using glucose isomerase to mixtures containing both fructose and glucose. Higher concentrations of fructose are particularly desirable and may be obtained by the use of more active enzymes and/or the use of high isomerization temperatures.
Detailed descriptions of the enzymatic conversion of glucose to fructose employing glucose isomerase can be found in Hamilton, et al. "Glucose Isomerase: a Case Study of Enzyme-Catalyzed Process Technology", Immobilized Enzymes in Food and Microbial Processes, Olson et al., Plenum Press, New York, (1974), pp. 94-106, 112, 115-137; Chen, et al., "Glucose Isomerase (a Review)", Process Biochem., (1980), pp. 36-41; Nordahl, et al., "Fructose Manufacture from Glucose by Immobilized Glucose Isomerase", Chem. Abstracts, vol. 82, (1975), Abs. No. 110316h; and Takasaki "Fructose Production by Glucose Isomerase", Chem. Abstracts, vol. 81, (1974), Abs. No. 7647a. In addition, there are numerous patents relating to glucose isomerization of which U.S. Pat. Nos. 3,616,221, 3,623,953 (Reissue 28,885), 3,694,313, 3,708,397, 3,715,276, 3,788,945, 3,909,354, 3,960,663, and 4,308,349 are representative.
Because of the economics involved in producing glucose isomerase, it is of the utmost importance to use the isomerase under conditions whereby maximum yields of fructose are produced using minimum quantities of glucose isomerase. Moreover, the conditions for isomerization should be such that minimal quantities of objectionable by-products are produced.